Support transparent PNG

src/activities/reader/XtcReaderActivity.h

@@ -29,4 +29,9 @@ class XtcReaderActivity final : public Activity {
   void loop() override;
   void render(RenderLock&&) override;
   bool isReaderActivity() const override { return true; }
+
+  // Renders the last saved page to the frame buffer without flushing to display.
+  // Used by SleepActivity to prepare the background for the overlay sleep mode.
+  // Returns false if the page cannot be loaded (missing cache / file error).
+  static bool drawCurrentPageToBuffer(const std::string& filePath, GfxRenderer& renderer);
 };